This is the clearest introduction to Set Theory I've read so far.
I read Jon Rahn's Basic Atonal Theory, which has some easy to read sections, and some nearly impossible for mere mortals sections, and I read the first half of Allen Forte's Structure of Atonal Music , which I found very difficult.
I'm so glad I found Straus! This book is helpful for not only atonal music, but also complex late-tonal music and I think it clarified my thinking about tonal music as well.